Meshes can now have multiple UV and vertex color layers, a long requested feature. A typical use of multiple UV layers would be to have one manually mapped layer for painting textures, and another automatically for baking.
The python API has been updated to support multiple layers.
The Mesh panel in the Editing buttons contains controls to create and delete layers, with an overview of existing layers. One layer is marked as active, and it is this layer that is then displayed and edited.
By default, materials will always use the active UV and vertex color layers. Using different UV layers in materials can be done in two ways. If the UV mapping type is chosen in the Map Input panel in the Material buttons, a field to specify the layer name is available. For node materials, the Geometry node contains a UV layer name field, and a new Vertex Color node has been added to access non active vertex color layers.
To add multiple UV and vertex color layers, and make it easier to add other types of layers to a mesh in the future, internally a system for custom data layers has been added.